Known fiber optic transmission systems have several inherent disadvantages. For example, known LED transmitters in transmission systems using fiber optic links are constructed to launch the amount of optical power required for worst case conditions. Typically, worst case conditions for such optical systems are computed using worst case receiver sensitivity, worst case LED optical power output, operation at worst case temperature and operation at worst case connector and film loss. Designing for such worst case conditions results in the launching of excess optical power (whenever conditions on a particular link are not worst case) which, in turn, results in an excess of dissipated power by the LED. Operating the LED at such continued high optical power results in an excessive amount of heat generated and may degrade LED performance over a period of time (as compared to operating at lower power levels).
Another disadvantage of known systems is that they provide little or no information with respect to connector performance in most applications. Further, knowledge of system expected power margins is uncertain due to the lack of information relating to installed connector performance. The availability of such information is particularly important in an airborne system.
Known systems are, for the most part, non-linear digital systems. Unlike the invention described herein, such known systems cannot measure noise performance in a fiber optic transmission and distribution system and relate such performance to power margins. This is because the relationship between noise power and bit error rate changes too rapidly near the threshold of transmission system operation. Presently, known systems simply transmit as much power as possible during any transmission. Maintenance of such systems is typically done only after a link in the system fails to operate.
The invention overcomes the disadvantages of prior art devices by providing, for the first time, apparatus which measures system noise performance and uses this information in one illustrative embodiment to control the amount of power launched or transmitted by an LED transmitter. As provided by the invention, an LED or laser transmitter launches only the amount of optical power required to maintain an adequate signal-to-noise ratio at the demodulator (discriminator) output. Reducing the launched power reduces the power dissipation of the transmitter and improves its reliability. Noise measurement results are also used by the invention to optimize transmission network route selection, and according to need (based upon measurements) do maintenance of the fiber optic transmission system only as required. Such maintenance can frequently be done prior to complete failure of optical fiber links. That is, the system employing the invention will transmit at a power level corresponding to actual conditions present in the system whereas known systems transmit at much higher power levels corresponding to "worst case" power loss calculations.
Optimization of receiver operating conditions can also be done if an Avalanche Photo Diode (APD) is used for an optical detector by using noise measurements to optimize APD gain. Conventionally, APD detectors use temperature compensated drive systems of the tightly regulated high voltage drive to control and optimize the avalanche gain of the devices.